HOUSTON - Anybody on Earth who’s ever moved into a new apartment and jammed a sofa into a too-narrow doorway appreciates the value of having a passageway that's big enough for your stuff. And that goes double in space, where the option of going outside and looking for another entrance isn’t practical.

Aboard the international space station, astronauts are now unloading several tons of supplies and equipment from the Leonardo cargo module, which was brought up on the space shuttle Discovery. They're also transferring a year’s worth of trash and recyclable equipment into the module's vacated slots.

In the process, they're using a particular element of the space station that usually gets overlooked amid all the attention to more sexy features such as solar panels, air locks and oxygen generators. It’s called the “hatch,” and it just doesn’t get any respect — despite the fact that its 4-foot-plus width gives the international space station capabilities that are unprecedented in 30 years of orbital operations, going back to NASA's Skylab and Russia's earliest outposts.

“The large hatch truly is an unsung hero of the ISS design!” said now-retired space station astronaut Dan Bursch, co-holder of the U.S. record for longest-ever space mission.

“I must say that I don't remember getting any bumps or scrapes from hitting the hatch or frame of the ISS hatch design,” he said in an e-mail to MSNBC.com, “but I sure do remember getting bumps and scrapes from the smaller ones!”

The transfer advantage
The benefit goes beyond mere comfort and convenience. “The ability to transfer that much big hardware has been a big advantage,” noted David McCann, the leader of the Boeing Co.’s Structures and Mechanical Systems Group at NASA's Johnson Space Center in Houston.

McCann said a smaller hatch would have had a negative impact on the design — and capability — of all the space station's internal equipment. Without it, “we couldn’t have built the [science] racks the way we did, or outfitted the space station the way we did,” he told MSNBC.com.

French cosmonaut Claudie Haignere and Russian cosmonauts Victor Afanasyev and Konstantin Kozeyev wave to the camera through a port between their Soyuz spacecraft and the international space station during a 2001 visit. The station's Soyuz tunnel is smaller than the American hatches.

Former astronaut Don Thomas, now chief scientist for the station program, said the 4-foot-wide hatch makes it possible to bring in equipment as big as a standard-size refrigerator with ease. “Without the large hatch that ISS provides, we would be limited to flying much smaller experiments, which would limit what we have been able to achieve on the ISS,” he said.

The hatch is part of what is formally called the Common Berthing Mechanism, a device installed at each end and often around the waist of U.S. modules such as the Unity connecting node and the Destiny laboratory.

The doorway is square, 51 inches (130 centimeters) on each side. This compares with the round tunnels from docking vehicles such as the Soyuz (31 inches, or 80 centimeters) and the space shuttle (42 inches, or 107 centimeters), whose docking tunnels must be tucked into rugged mating mechanisms. In terms of actual cross-sectional area, the shuttle tunnel has twice the area of the Soyuz tunnel, and the Common Berthing Mechanism's door has twice again the area of the shuttle tunnel.

Historically tiny tunnels
Smaller diameters have been the historical norm. For example, back in the late 1960s and early '70s, the tunnel between the Apollo command module and the lunar module was a mere 29 inches wide. Even, now, the Russians are going with the 31-inch width for their next-generation Kliper spacecraft. And because the European Space Agency's brand-new Automated Transfer Vehicle has to link up with Russian hatches, that cargo craft will have to follow the 31-inch standard as well.

People can slide through these smaller tunnels, their shoulders brushing the walls and the hatch rims. Small packets, or golf-bag sized gadgets, can be wrestled in and out. But anything bigger would have to be installed in the space station pre-launch, and stay there forever. This explains why many of the bulkier Russian items that need replacement, such as the Elektron oxygen generators, are long cylinders. That’s the biggest size and shape that can get inside.

On the now-defunct Mir space station, the Russians became overwhelmed with bulky equipment that had broken down or became obsolete, and then couldn’t be discarded. At the end of the station’s life in 2001, the best argument for terminating its flight was that very little of the onboard equipment actually worked at all — but was taking up all the space aboard.

On the U.S. side of the international space station, the size and shape of the hole is matched with the size and shape of the biggest item planned to pass through it — the refrigerator-sized racks. These racks can contain sophisticated scientific equipment, or life-support devices, or even just portable closets.

The modules can be brought on board from the cargo containers, or taken back into the modules for return to Earth, servicing and eventually reinstallation in orbit. They are the heart of the research programs for which the station is being built — and without the right-sized door, they would be stuck on Earth.

It's not a simple matter to build a door that big for space. McCann explained that the pressurization inside the space station puts 20 tons of force on the hatch. Also, the fact that the door is square means the stress isn't distributed uniformly around the edges. For those reasons, the hatch has to be exceptionally strong and stiff.

“It was a challenge to design,” McCann admitted, “but the hatch and the CBM have worked flawlessly on orbit.”

Scientific payoff
Don Thomas, the station's mission scientist, said eight racks' worth of scientific experiments have been transferred from the shuttle to the station so far.

“These facilities have allowed us to conduct a wide range of scientific investigations on how the human being adapts to the microgravity environment ... and have supported a wide range of physical science investigations from crystal growth to the development of advanced materials with unique structures and properties,” he said in his e-mail.

During Discovery's mission, two new research racks are being delivered: a laboratory freezer that can preserve biological samples at a temperature of 112 degrees below zero Fahrenheit (-80 degrees Celsius), and a European incubator that will be used for plant and animal studies.

The space-to-space passageway isn’t finished yet, Thomas said: “On future shuttle missions, six additional scientific racks will be flown including ones that will investigate crystal growth and materials processing in space, combustion processes, and one that will aid in our photography and investigations of the Earth from the ISS.”

Once the shuttle stops flying after 2010, the challenge will be to find new ways to use the plus-size doorway. Perhaps a new cargo carrier can be equipped with compatible mechanisms. Or perhaps a new airlock module can have its own door, big enough for bulky objects.

But the current Common Berthing Mechanism has surplus capability, even now. According to McCann, if needed, the hatch could be opened directly to space, once the air has been released from the Unity module. “If we absolutely had to get something that large into the station, we could,” McCann said.

The 12 orbiting astronauts on the International Space Station were hailed by the chief on Thursday — and also got a humorous scolding for keeping their android crewmate locked up in its packing for so long.